In the olden days they used to believe that cutting a vein and letting you bleed for an hour or two would cure all sorts of maladies, while as recently as the last century some experts still believed nicotine was good for the nerves.
But don't laugh at our ignorant ancestors. We're just as guilty of believing medical myths, even when there isn't a shred of evidence to back them up. Here are 10 that refuse to die.
Men need to drink 2.5 litres of water a day
We've been told for years that men need to drink about eight glasses - or 2.5 litres - of water a day, a figure that partly explains the explosion in popularity of bottled water over the last decade or two. The only problem is, it's complete medical myth.
The fact is, our bodies are brilliant at regulating water intake and will tell you they need to take in more using a handy sensation called thirst. The exact amount depends on temperature, our activity levels, how much water is in the food we've eaten and so on.
There's no serious evidence that drinking more water than your thirst demands is good for you, and none that bottled water is better than tap.
Crunches get rid of belly flab
It seems common sense - to get rid of belly fat, work on your belly. Specifically, exercises like crunches will turn belly fat into muscle. Job done.
In fact, in a recent study by Duke University Medical Center in the US, aerobic exercise like jogging beat resistance training hands down when it came to battling belly bulge.
We only use 10% of our brains
It's common in self-help books to claim - as a way of showing how much more we all have to offer - that we only use 10% of our brains. The myth is widely believed, and has been for over a century. It was also once considered truer for men, whose more practical and unemotional approach meant that their mental activity was seen as even more limited than that of women.
The truth is that damage to any part of the brain leads to lasting problems, showing that all parts of the brain are utilised. Brain imaging studies have also failed to find any part of the brain that is completely inactive. We may not use all our brain power, but we certainly use most of it.
Shaving causes stubble to grow back faster and thicker
Every schoolboy knows that if you quickly shave off the first sight of bum fluff you find on your chin, you'll soon be sporting thick manly stubble. But then schoolboys are wrong about a lot of things.
And so are grown men, it seems. This popular myth was disproved as far back as 1928, when a study showed that shaving had no effect on hair growth. The myth might have come about because new stubble has not had chance to be lightened by the sun or chemicals in the environment, and darker hair can look thicker.
Reading in dim light ruins your eyesight
It's true that reading in dim light can cause eyestrain and dryness, and that it can make your eyes feel uncomfortable. Dim light can also make it harder to focus on the words in front of you. Put those things together and people have reached the conclusion that reading in dim light ruins your eyesight.
On the other hand, the vast majority of expert opinion says that such a conclusion is, well, bunkum. In reality, the problems are temporary and rectify themselves as soon as you put down the book or turn on a lamp. It's also worth remembering that before the electric light, people commonly read by candlelight, with no apparent detrimental impact on their eyesight.
Mobile phones stop medical equipment working
The reason most people think there are signs in hospitals asking you to turn off your mobile phone is that they interfere with crucial, lifesaving hospital equipment. And indeed they can, very rarely, and if they're very close to the equipment in question.
In normal circumstances, mobile phones have no effect on hospital equipment. In 2007 a large study showed that mobile phones used in a 'normal way' caused no malfunction or interruption of any kind during 300 tests in 75 treatment rooms. The reason many hospitals want you to switch mobile phones off is to stop your conversations annoying other patients.
Hair and fingernails continue to grow after death
This common myth certainly seems to be true. After death, hair and fingernails can appear to be longer.
But it's an optical illusion. Dehydration of the body after death causes the skin to retract, giving hair and fingernails the appearance of growth. But growing requires complex bodily systems to be working, and after death they simply aren't.
Coffee will sober you up
It used to be quite normal for sozzled revellers to be offered a cup of black coffee to sober them up. It's less common now, for the simple reason that it doesn't work.
A male liver can only deal with about one normal sized alcoholic drink (a pint of normal strength beer, for example) an hour. No amount of coffee, orange juice or cola will make any difference to that. You'll stay drunk till your liver has metabolised much of the alcohol in your blood.
Alcohol does make you sleepy, and the caffeine in coffee (or cola) will perk you up a bit, so that's where this myth may have started.
Cold weather gives you a cold
It seems too obvious to contradict, and surely it's called a cold because you get it by going out in cold weather?
In fact, viruses cause colds, not the weather. To catch one, you have to be in the vicinity of someone who has already succumbed. We catch more colds in winter because we huddle together inside for more of the time, making person-to-person transition easier. Temperature has nothing directly to do with it.
Cracking your knuckles causes arthritis
It's one of the stranger manly quirks - that thing many of us do where we pull our fingers away from their sockets until the knuckles make a cracking sound. It was all the rage, right up until someone told us it could cause arthritis.
Well, we can go back to the knuckle-cracking, lads, because the arthritis thing is a myth. When your knuckles 'crack' it's simply the sound of nitrogen bubbles in the lubricating fluid between your joints bursting. This can cause some damage if you do it too much, but it has nothing to do with arthritis, which occurs when the body's immune system attacks its own joints.
So that's 10 medical myths that won't go away, and even doctors still believe some of them. Can you think of any others?
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